Brewer's yeast is used as a biocatalyst to ferment carbohydrates to ethanol in the production of beer and other fermented beverages. In brewing, fermentation is performed by mixing brewer's yeast with wort and incubating the mixture under conditions suitable for fermentation. Wort includes a source of carbohydrates prepared by hot water enzymatic conversion of complex sugars to fermentable sugars in malted grain and adjunct grains. After extraction the wort is boiled to complete extraction, stop enzymatic reactions, and to boil off undesirable compounds. Following boiling the wort is cooled and brewer's yeast is added.
During the brewing process, measuring the specific gravity over time monitors fermentation. The specific gravity declines over the course of fermentation due to a decrease in fermentable carbohydrates and an increase in ethanol concentration. What constitutes an acceptable final specific gravity depends on the type of beer being brewed.
An unacceptably high specific gravity may be caused by a variety of factors, including insufficient aeration of the wort, low concentration of yeast in the inoculum, poor yeast growth due to insufficient nutrients, unsuitably high or low fermentation temperatures, excessive yeast growth, and the like. Some fermentation processes are characterized by a low initial fermentation rate (caused by poor initial yeast growth or inadequate inoculum) and an increase in fermentation over time.
Within the brewing industry, there is considerable interest in increasing the rate of fermentation. Increased fermentation rates not only reduce fermentation time, but also reduce the incidence of brewing failures due to contamination by microorganisms, which may result in an unacceptably poor quality product.
Fermentation may be enhanced by a variety of known means. For example, U.S. Pat. Nos. 899,756; 1,041,298; and 2,121,458 teach that aeration of the wort/yeast solution can assist in the growth of yeast to promote fermentation. Belgium Patent 1010885A3 teaches that the aeration of the wort/yeast suspension may be achieved by using a porous membrane. However, it is believed that aerating the wort/yeast solution may contribute to the production of staling precursors (Uchida, et al., J. Am. Soc. Brew. Chem. 58(1):30-37 (2000)). Another approach to enhancing fermentation is by adding zinc to the yeast/wort solution as disclosed in U.S. Pat. Nos. 3,164,472 and 4,840,802.
Another way in which the fermentation rate may be increased is by increasing the pitching rate. Adequate pitching reduces lag time and reduces the likelihood that a bacterial contaminant will become established. The pitching rate may be increased by increasing the number of yeast cells added to the wort, or by using a yeast starter culture.
A starter culture may be made by first inoculating a smaller volume of wort with an active yeast followed by vigorous aeration/agitation which allows for the concentration of active yeast cells to increase before pitching the starter culture into a larger volume of wort.
Commonly, wort is pitched with yeast derived from a previous fermentation. Generally, this yeast has experienced anaerobic conditions during fermentation. Before fermentation can occur, the yeast which is harvested from the anaerobic beer environment and is used to pitch the wort must be supplied oxygen in order to synthesize essential lipid components, including sterols and unsaturated fatty acids. Synthesis of these lipids requires molecular oxygen and a carbohydrate source, such as glycogen, stored in the yeast cells.
A conventional approach to insuring sufficient oxygen for yeast to synthesize lipids has been to oxygenate the wort. However, the level of oxygen in the wort must be controlled to avoid slow fermentation and subsequent flavor changes caused by sub-optimal concentrations of oxygen, and reduced ethanol yields and flavor changes that result from excessive yeast growth and metabolic changes caused by high levels of oxygen.
Another approach to enhancing fermentation rates is to pitch wort containing no oxygen or reduced oxygen with a starter culture of yeast prepared by allowing yeast to grow with exposure to oxygen in a smaller volume of wort for several hours. This method allows control of fermentation by controlling the pitching rate.
UK Patent Application GB 2 197 341 discloses a method of fermenting wort in which the pitching yeast is first suspended in water and exposed to oxygen for a period of time until the yeast reaches its maximum rate of oxygen consumption. The yeast is then used to pitch oxygen-free wort.
There remains a need in the art for improved methods of enhancing fermentation.